Two-slope soft X-ray spectra observed in experiments on electron cyclotron resonance plasma heating in the L-2M stellarator

In experiments on the generation and electron cyclotron resonance heating (ECRH) of plasma in the L-2M stellarator, non-Maxwellian two-slope soft X-ray (SXR) spectra were observed. The temperatures of the thermal and epithermal components of the spectra were measured as functions of the heating power and plasma density. A hypothesis based on the experimental results is suggested to explain the formation mechanism of two-slope SXR spectra in the ECRH experiments at the L-2M stellarator. The measured SXR spectra are compared with the results of numerical simulations.     

Specific features of the structure of the Z-pinch emitting region formed during the implosion of a foam-wire load at the ANGARA-5-1 facility

Results are presented from experimental studies of the structure of the compressed plasma of a Z-pinch produced during the implosion of a foam-wire load at the current of up to 3 MA. The foam-wire load consisted of two nested cylindrical cascades, one of which was a solid or hollow cylinder made of low-density agar-agar foam, while the other was a wire array. The wall thickness of a hollow foam cylinder was 100–200 μm. The images of the pinch and its spectrum obtained with the help of multiframe X-ray cameras and a grazing incidence spectrograph with a spatial resolution were analyzed. Data on the spatial structure of the emitting regions and the soft X-ray (SXR) spectrum of the Z-pinch in the final stage of compression of a foam-wire load were obtained. The implosion modes characterized by the formation of hot regions during implosion of such loads were revealed. The characteristic scale lengths of the hot regions were determined. It is shown that the energy distribution of SXR photons in the energy range from 80 eV to 1 keV forms the spatial structure of Z-pinch images recorded during the implosion of foam-wire loads. It is revealed that the spectral density of SXR emission in the photon energy range of 300–600 eV from hot Z-pinch regions exceeds the spectral density of radiation from the neighboring Z-pinch regions by more than one order of magnitude. Groups of lines related to the absorption and emission of radiation by atoms and multicharged ions of carbon and oxygen in the outer foam cascade of a foam-wire load were recorded for the first time by analyzing the spatial distribution of the SXR spectra of multicharged ions of the Z-pinch. The groups of absorption lines of ions (C III, O III, O IV, and O VI) corresponding to absorption of SXR photons in the Z-pinch of a tungsten wire array, which served as the inner cascade of a foam-wire load, were identified. The plasma electron temperature measured from the charge composition of carbon and oxygen ions in the outer agar-agar foam cascade was 10–40 eV. During the implosion of foam-wire loads at currents of up to 3 MA, SXR pulses (hν > 100 eV) with a duration of 10 ns and peak power of 3 TW were detected. It is shown that the temporal profile of single-peak and double-peak SXR pulses can be controlled by varying the parameters of the outer and inner cascades of the foam-wire load.     

Influence of the plasma density and heating power on the intensity of electron cyclotron emission in the L-2M stellarator

Results are presented from experiments on studying the plasma behavior in the L-2M stellarator in regimes with a high power deposition in electrons during electron cyclotron heating at the second harmonic of the electron gyrofrequency (X mode) at heating powers of P _in=120–400 kW and average plasma densities from n _e≤3×10¹⁹ to 0.3×10¹⁹ m^?3. It is shown that, as the plasma density decreases and the heating power increases, the electron cyclotron emission spectrum is modified; this may be attributed to a deviation of the electron energy distribution from a Maxwellian and the generation of suprathermal electrons. At low plasma densities, the emission intensity at the second harmonic of the electron gyrofrequency increases, whereas the plasma energy measured by diamagnetic diagnostics does not increase. This poses the question of the correctness of determining the plasma electron temperature by electron cyclotron emission diagnostics under these conditions.     

Numerical simulations of ECE spectra under the L-2M experimental conditions

Results are presented from numerical simulations of electron-cyclotron emission spectra from the plasma of the L-2M stellarator under the conditions corresponding to experiments on electron-cyclotron resonance heating. The spectra are calculated in the model of a two-component electron plasma, each component (thermal and suprathermal) having a Maxwellian distribution. It is shown that, even when suprathermal electrons contribute insignificantly to the total plasma energy, they may considerably affect both the shape and intensity of the electron-cyclotron emission spectrum. The occurrence of a high spectral peak at the frequency corresponding to the resonance ω = 2ω_ce on the low-field side of the plasma column in regimes with a high specific heating power is explained.     

Probe measurements of the electron distribution function in an electron-beam-produced ytterbium plasma

A nonequilibrium anisotropic plasma produced by an electron beam in the residual air with a low content of ytterbium vapor was investigated by the probe method. It is found that a minor (at a level of a few ppm) admixture of ytterbium to low-pressure air substantially modifies the electron energy distribution function (EEDF): the main peak corresponding to thermal electrons broadens, and new peaks appear. It is shown that the observed change in the EEDF is caused by the low ionization energy of ytterbium, due to which one beam electron can ionize several ytterbium atoms. The new peaks in the EEDF correspond to the final energies of a beam electron after each subsequent ionizing collision with ytterbium atoms.     

Effect of the shape of the electron energy distribution function on the dust grain charge and its screening in glow discharge plasmas

The dust grain charge in the plasma of a glow discharge in noble gases and nitrogen is calculated in the orbit motion limited model for reduced fields in the range of E/N = 1–20 Td. The calculations were performed using the electron energy distribution functions (EEDFs) obtained by solving the Boltzmann equation numerically with allowance for elastic and inelastic electron scattering and analytically with allowance for only elastic scattering and (for nitrogen) excitation of rotational levels, as well as using a Maxwellian EEDF. In the latter case, either the characteristic electron energy or mean electron energy multiplied by two thirds was used as the electron temperature. It is shown that the calculations with the use of a Maxwellian EEDF yield larger values of the grain charge as compared to those calculated with EEDFs obtained by solving the Boltzmann equation. The range of E/N values is determined in which analytical expressions for the EEDF obtained with allowance for elastic scattering and excitation of rotational levels are applicable to calculating the grain charge. The effect of the EEDF shape on the screening of the dust grain charge in plasma is investigated. The Debye screening length in case of a Maxwellian EEDF is shown to be shorter than that obtained with EEDFs calculated by numerically solving the Boltzmann equation.     

Implosion dynamics of condensed Z-pinch at the Angara-5-1 facility

The implosion dynamics of a condensed Z-pinch at load currents of up to 3.5 MA and a current rise time of 100 ns was studied experimentally at the Angara-5-1 facility. To increase the energy density, 1- to 3-mm-diameter cylinders made of a deuterated polyethylene?agar-agar mixture or microporous deuterated polyethylene with a mass density of 0.03–0.5 g/cm³ were installed in the central region of the loads. The plasma spatiotemporal characteristics were studied using the diagnostic complex of the Angara-5-1 facility, including electron-optical streak and frame imaging, time-integrated X-ray imaging, soft X-ray (SXR) measurements, and vacuum UV spectroscopy. Most information on the plasma dynamics was obtained using a ten-frame X-ray camera (Е > 100 eV) with an exposure of 4 ns. SXR pulses were recorded using photoemissive vacuum X-ray detectors. The energy characteristics of neutron emission were measured using the time-offlight method with the help of scintillation detectors arranged along and across the pinch axis. The neutron yield was measured by activation detectors. The experimental results indicate that the plasma dynamics depends weakly on the load density. As a rule, two stages of plasma implosion were observed. The formation of hot plasma spots in the initial stage of plasma expansion from the pinch axis was accompanied by short pulses of SXR and neutron emission. The neutron yield reached (0.4–3) × 10¹⁰ neutrons/shot and was almost independent of the load density due to specific features of Z-pinch dynamics.     

Study of the expansion dynamics of the plasma of a thin aluminum foil under the action of soft X-ray emission

Results are presented from experiments on the irradiation of thin aluminum foils by an intense soft X-ray (SXR) source on the basis of the Z-pinch formed during the implosion of a tungsten wire array at the Angara-5-1 facility. The state of the foil target is examined by taking two-dimensional X-ray frames. The expansion velocity of the plasma formed under the action of pulsed SXR emission on the front (irradiated) and back sides of the foil and the glow intensity of aluminum plasma on the back side are found from the spatial distribution of the radiation intensity of the plasma of the irradiated foil. The time at which the foil plasma becomes transparent to Z-pinch radiation is determined from the increase in the intensity of transmitted SXR emission.     

Generation and anisotropy of neutron emission from a condensed Z-pinch

The paper presents results of measurements of neutron emission generated in the constriction of a fast Z-pinch at the S-300 facility (2 MA, 100 ns). An increased energy concentration was achieved by using a combined load the central part of which was a microporous deuterated polyethylene neck with a mass density of 100 mg/cm³ and diameter of 1–1.5 mm. The neck was placed between two 5-mm-diameter agar-agar cylinders. The characteristics of neutron emission in two axial and two radial directions were measured by the time-of-flight method. The neutron spectrum was recovered from the measured neutron signals by the Monte Carlo method. In all experiments, the spatiotemporal characteristics of plasma in the Z-pinch constriction were measured by means of the diagnostic complex of the S-300 facility, which includes frame photography in the optical, VUV, and soft X-ray (SXR) spectral regions; optical streak imaging; SXR detection; and time-integrated SXR photography. The formation of hot dense plasma in the Z-pinch constriction was accompanied by the generation of hard X-ray (with photon energies E > 30 keV), SXR (with photon energies E > 1 keV and duration of 2–4 ns), and neutron emission. Anisotropy of the neutron energy distribution in the axial direction was revealed. The mean neutron energies measured in four directions at angles of 0° (above the anode), 90°, 180° (under the cathode), and 270° with respect to the load axis were found to be of 2.1 ± 0.1, 2.5 ± 0.1, 2.6 ± 0.2, and 2.4 ± 0.1 MeV, respectively. For a 1-mm-diameter neck, the maximum integral neutron yield was 6 × 10⁹ neutrons. The anisotropy of neutron emission for a Z-pinch with a power-law distribution of high-energy ions is calculated.     

Studies of the distribution of the Z-pinch radiance during the implosion of wire arrays in the angara-5-1 facility

Results are presented from time-integrated measurements of soft X-ray emission from Z-pinches during the implosion of simple and nested wire arrays. The blackening density distribution obtained with the help of a pinhole camera is recalculated into the time-integrated Z-pinch radiance. It is found that, in the case of a simple wire array, up to 70% of the total SXR energy emitted during a discharge is radiated from the axial region, the rest of energy being radiated from plasma jets, whereas in the case of a nested wire array, more than 90% of the SXR energy is radiated from the axial region.     

The dynamics of and radiation from a copper-wire corona in a megaampere Z-pinch

Evolution of the extreme ultraviolet (XUV) and soft X-ray (SXR) emission in the 50-to 2000-eV photon energy range from a plasma corona formed by loading a relatively thick Cu wire (with an initial diameter of 120 µm) was observed in a Z-pinch discharge with a maximum current of 2 MA and current rise time of 100 ns. A diagnostic complex consisting of a five-channel SXR polychromator, a four-frame X-ray pinhole camera, and a mica crystal spectrograph shows that double-humped emission pulses in the XUV and SXR spectral ranges are generated 70–130 ns after the onset of the discharge current. The total energy of the pulses is 5 kJ, and the maximum power is 60 GW. A part of the observed kiloelectronvolt X-ray emission from three to five spots with diameters of 1–2 mm consists of the Cu K-and L-shell lines.     

Deviation of the electron energy distribution function from maxwellian during ECR heating in the L-2M stellarator in regimes with a high specific heating power

In the previous experiments on ECR heating of a low-density plasma with n _e =(0.3?0.5)×10¹⁹ m^?3 in the L-2M stellarator, the electron temperature profile measured from the intensity of electron cyclotron emission was found to be asymmetric about the magnetic axis and the electron temperature measured by this diagnostics turned out to be higher than that expected from diamagnetic measurements. To find out the character of distortion of the electron energy distribution function, the soft X-ray spectrum was measured in regimes with large values of the specific heating power η (1.5 MW per 10¹⁹ particles). Under these conditions, the X-ray spectrum plotted on a semilogarithmic scale has no linear segments in the photon energy range from 1.5 to 12 keV. This indicates that the electron distribution function is non-Maxwellian over the entire energy range under study.     

Somatic Antigens of Streptococcus Group E: I. Comparison of Extraction Techniques1

Eleven Streptococcus group E strains, representing serotypes I, II, III, IV, V, and "untypable" isolates, were extracted by formamide, trichloroacetic acid, and hydrochloric acid under various conditions in an effort to determine the best method for recovering maximum amounts of group and type antigens. The group antigen was found to be relatively stable, and adequate amounts for identification purposes were recovered by a wide spectrum of conditions. Type-specific antigens were relatively labile, and were destroyed at low pH in acid hydrolysis or by prolonged heating in formamide hydrolysis. The best single procedure for recovering both type and group antigens from Streptococcus group E was formamide hydrolysis for 30 min at 180 C.     

Electron cyclotron resonance plasma heating in the CERA-RX facility under a randomly pulsating electric field

The results of the numerical simulation of the electron cyclotron resonance (ECR) heating of plasma particles in the CERA-RX facility under a randomly pulsating electric field (the collective effects taken into account) are presented. Under these conditions, the electron energy spectrum was found to be depleted to the low energy region due to an increase in the number of particles in the high energy region. The obtained effect depends on the polarity of the pulsating electric field.     

Particle diagnostics of ion cyclotron resonance plasma heating in the Globus-M tokamak

Basic experimental results on cyclotron heating of the ion plasma component in the Globus-M spherical tokamak obtained by means of the ACORD-12 charge-exchange ion analyzer are presented. A procedure for determining the maximum energy of fast ions confined in the plasma is described. The procedure was applied to estimate the limiting energy of hydrogen minority ions accelerated during ion cyclotron heating in the Globus-M tokamak. The experimental evaluation of the maximum hydrogen ion energy is confirmed by simulations of ion orbits. Recommendations for optimizing experiments on ion cyclotron heating in the Globus-M tokamak are formulated.     

Hard X-ray emission from imploding wire arrays

Results are presented from experimental studies of the generation of hard X-ray (HXR) emission with photon energies above 20 keV during the implosion of wire arrays in the Angara-5-1 facility. An analysis of X-ray images of the Z-pinch shows that the dimensions and spatial structures of the emitting regions are different for hard and soft X rays. It is found that the HXR emission peak is delayed with respect to the soft X-ray (SXR) one. The dependence of the HXR power on the material, initial diameter, and mass (implosion time) of the wire array is determined. It is shown that the HXR intensity in the spectral range >50 keV is several orders higher than the emission intensity in the high-energy tail of the SXR spectrum (assuming that this spectrum is thermal). A comparison of the time evolution and spatial localization of the HXR and SXR sources during the implosion of wire arrays indicates the presence of a new superthermal phenomenon that differs qualitatively from the processes determining the peak power of the SXR pulse. Possible mechanisms that can be responsible for the generation of HXR pulses are considered.     

Influence of the pulsating electric field on the ECR heating in a nonuniform magnetic field

According to a computer simulation, the randomized pulsating electric field can strongly influence the ECR plasma heating in a nonuniform magnetic field. It has been found out that the electron energy spectrum is shifted to the high energy region. The obtained effect is intended to be used in the ECR sources for effective X-ray generation.     

High-Power X-Ray Line Radiation of the Plasma Produced in a Collision of High-Energy Plasma Flows

Results are presented from experimental studies of a pulsed source of soft X-ray (SXR) emission with photon energies in the range of 0.4–1 keV and an output energy of 2–10 kJ. SXR pulses with a duration of 10–15 μs were generated in collisions of two plasma flows propagating toward one another in a longitudinal magnetic field. The plasma flows with velocities of (2–4) × 10⁷ cm/s and energy contents of 70–100 kJ were produced by two electrodynamic coaxial accelerators with pulsed gas injection. Nitrogen and neon, as well as their mixtures with deuterium, were used as working gases. The diagnostic equipment is described, and the experimental results obtained under different operating conditions are discussed. In particular, X-ray spectroscopy was used to study the high-temperature plasma produced in a collision of two plasma flows. The observed intensities of spectral lines are compared with the results of detailed kinetic calculations performed in a steady-state approximation. The calculations of the nitrogen and neon kinetics have shown that the electron temperature of a nitrogen plasma can be most conveniently determined from the intensity ratio of the resonance lines of He- and H-like nitrogen ions, while that of a neon plasma, from the intensity ratio between the resonance line of He-like Ne IX ions and the 3p?2s line of Li-like Ne VIII ions. In the experiments with plasma flows containing nitrogen ions, the electron temperature was found to be ≈120 eV, whereas in the experiments with plasma flows containing neon ions, it was 160–170 eV.     

Spatial distribution of the plasma temperature under ion-beam fast ignition

Results are presented from theoretical studies of the formation of the spatial temperature distribution in plasma heated by a high-energy ion beam under the conditions in which the free path lengths of ions of different parts of the beam in plasma varies in the course of its heating. Special attention is paid to ionbeam heating of deuterium-tritium (DT) plasma under the conditions of fast ignition of inertial confinement fusion (ICF) targets. The influence of the initial energy spectrum of the heating beam ions on the spatial temperature distribution is investigated. For beams with different ion charges, masses, and initial energy spectra, criteria are determined for the formation of different types of spatial temperature distributions, namely, a distribution with a negative temperature gradient and a quasi-uniform distribution, which correspond to the edge ignition of a precompressed ICF target, as well as a distribution with a temperature peak, which corresponds to the ignition in the inner region of the target.